The present invention relates to a method for forming a developed image composed of a toner, the magnetic toner directly on the surface of a recording medium, using a large number of needle electrodes in accordance with electrical signals corresponding to the image.
Among the conventional image-forming methods with electro-photographic apparatus, the most general method is one which comprises using as an image carrier, for example, a photosensitive drum, uniformly charging the surface of the drum by a corona-charging means, exposing the drum to light to form an electrostatically charged image on the surface thereof, developing the image with a magnetic developer by the magnetic brushing method, etc., transferring the developed image to a recording member and then fixing the transferred image.
In recent years, however, a means has been proposed in which visible images composed of a toner are formed directly on a recording medium comprising Alumite or other materials by means of plural number of needle electrodes without using the foregoing photosensitive drum (for example refer to U.S. Pat. No. 3,816,840). For example, the outer surface of an aluminum drum is covered with Alumite layer of about 10 .mu.m in thickness, a permanent magnet is provided near to the inner surface of the drum, and a toner container containing conductive magnetic toner is arranged over the outer surface of the drum so that it faces the foregoing permanent magnet. At a part of the toner container are provided a magnetic blade and plural number of needle electrodes so that they face the toner and permanent electrode. Toner chains are formed between the Alumite layer and needle electrodes by the action of magnetic field of the permanent magnet, and a part of the toner chains is brought into contact with the Alumite layer. In a system constituted as above, when electrical signals corresponding to an image, for example, pulse voltage of about 50 V are selectively applied to plural number of the needle electrodes, coulomb force acts on the toner in contact with the Alumite layer. When the drum is rotated while the coulomb force is acting, the toner selectively adheres to the Alumite layer constituting the outer surface of the drum, thus developing is carried out. Consequently, a copied image can be obtained thereafter by electrostatically transferring the developed image to plain paper, etc. by a usual means and fixing the transferred toner image.
When the visible image composed of the toner is directly developed on the, dielectric as described above, various problems are caused by using the conventional magnetic toners as they are. For example, when conductive magnetic toners (for example refer to U.S. Pat. Nos. 3,639,245, 4,189,390 and 4,482,623) are used, because of their resistance value (bulk resistance) being low, for example, about 10.sup.2 to about 10.sup.8 .OMEGA..cm, there is a problem of the image bleeding at the time of transferring. On the other hand, when semiconductive or insulating magnetic toners are used, because of their resistance value being high, for example, about 10.sup.9 to about 10.sup.16 .OMEGA..cm, there is a problem of the image density being low at the time of developing. Also, it is thought to reduce the internal resistance by incorporating conductive fine particles such as carbon black into the particles of the high-resistance magnetic toners. In this case, however, there is a necessity to incorporate a large quantity of carbon black, and so there is a problem of the property of fixing images (hereinafter referred to as fixability) being remarkably deteriorated.
As described in U.S. Pat. No. 4,873,540, therefore, it is proposed to use a magnetic toner comprising conductive fine particles and an insulating substance which adheres to the surface of toner particles. High-quality images are obtained by using this magnetic toner, but a further improvement in the fixability is desired.